ProjectSummary Retrotransposons,mainlyLINEs,SINEs,andendogenousretroviruses,occupy40%ofthemammalian genome.Retrotransposonshaveanenormouscapacitytometastasizethroughoutthegenomeusinga?copy andpaste?mechanisminvolvingreversetranscription.Whileretrotransposonsplayanimportantrolein genomeevolution,theirmobilizationcanbedetrimentaltogenomeintegrity.Indeed,morethan60human geneticdiseasesarecausedbytransposoninsertion.Retrotransposonsexploitthehostcellularmachineryto proliferate.Inresponse,thehosthasevolvedmultiplemechanismstosuppressretrotransposonstoprotect genomeintegrity,particularlywithinthegermline.ThepiRNApathwayisamajorevolutionarilyconservedsmall non-codingRNA-basedsilencingmechanismforretrotransposonsingermcells.Inthepreviousfundingperiod, wedemonstratedthatMOV10L1,agermcell-specificRNAhelicase,isamasterregulatorofbiogenesisofall piRNAsinmouse.MOV10L1interactswithallPiwiproteinsandbindstopiRNAprecursorstoinitiatepiRNA biogenesis.DeficiencyofMov10l1leadstoupregulationofretrotransposons,ablockinmeiosis,andmale sterility.Upregulationofretrotransposontranscriptsdoesnotnecessarilyleadtoaproportionateincreasein newretrotransposition,suggestingthatadditionalhostfactorsblockretrotransposition.Whilepreviousstudies havemadetremendousprogressdelineatingmechanismsresponsiblefortranscriptionalandpost- transcriptionalsilencingofretrotransposons,hostrestrictionfactorsthatpreventgenomicintegrationof retrotransposonsinvivohavenotyetbeenidentified.Usingouruniquemousemodels,weplanto1) investigatethemolecularmechanismunderlyingtheessentialroleofMOV10L1inpiRNAbiogenesisduring spermatogenesis;?2)elucidatethecriticalroleofahostrestrictionfactorininhibitionofretrotranspositioninthe mousegermline;?3)interrogatethemulti-generationalimpactofretrotransposon-drivengenomeexpansionon genomestability,reproduction,anddiseases.Completionofthisprojectwillhavestrongimpactsonour understandingofretrotransposonsilencing,genomeexpansion,andetiologyofhumandiseasesincluding maleinfertility,pregnancyloss,andbirthdefects.